Measurement of Intraocular Pressure
The term “glaucoma” encompasses a group of diseases, which cause progressive damage to the optic nerve and resultant optical field defects, vision loss and, in some cases, blindness. Typically, glaucoma is frequently, but not always, accompanied by abnormally high intraocular pressure.
There are three basic types of glaucoma—primary, secondary and congenital. The most common type of glaucoma is primary glaucoma. Cases of primary glaucoma can be classified as either open angle or closed angle.
Secondary glaucoma occurs as a complication of a variety of other conditions, such as injury, inflammation, vascular disease and diabetes.
Congenital glaucoma is elevated eye pressure present at birth due to a developmental defect in the eye's drainage mechanism.
Glaucoma is the third most common cause of blindness in the United States. Whether it is an increase in the intraocular pressure that causes damage to the retina or an increased susceptibility to damage that may result in an increase in intraocular pressure, titrating the intraocular pressure with careful monitoring is the mainstay of treatment and constitutes an important component in the overall clinical management of the disease.
The etiology of vision loss in glaucoma patients may be due, at least in part, to compression of the vasculature of the retina and optic nerve as a result of increased intraocular pressure. Indeed, it is generally accepted that controlling intraocular pressure through the use of drugs and/or surgery markedly reduces glaucomatous progression in normal-tension glaucoma and decreasing intraocular pressure virtually halts it in primary open-angle glaucoma. Furthermore, it is generally acknowledged that lowering intraocular pressure in glaucoma patients can prevent or lessen the irreversible glaucoma-associated destruction of optic nerve fibers and the resultant irreversible vision loss.
Thus, irrespective of the particular type of glaucoma a patient suffers from, it is typically desirable to obtain periodic measurements of intraocular pressure in order to assess the clinical progression of the disease and/or the efficacy of the treatments being administered. Also, because early diagnosis is important in effectively treating glaucoma, it is also desirable to periodically measure intraocular pressure in patients who do not presently suffer from glaucoma but who may be at risk to contract one of the various types of glaucoma.
Today, intraocular pressure is commonly measured by indirect methods (e.g., pressing a strain gage against the cornea and measuring the depth of corneal depression) or by non-contact methods (e.g., expelling a puff of air against the outer surface of the cornea and measuring the depth of corneal depression). As convenient as these measurements may be, they are inherently inaccurate, mainly because of the error imparted by the varying mechanical properties of the cornea. It has been shown that such indirect intraocular pressure measurements are dependent upon, among other factors, corneal thickness, curvature and rigidity. These factors can vary greatly from individual to individual, and thus gross errors in intraocular pressure estimation are common. These errors can easily result in the misdiagnosis of a glaucomatous or non-glaucomatous state. Moreover, with the advent of corneal refractive surgery, 1.8 million of which were performed in the U.S. last year, measurement of intraocular pressure via indirect methods through the cornea is even more inaccurate secondary to the biomechanical alterations of the cornea caused by surgery. Thus there is a great national and international need to develop a more accurate direct intraocular pressure sensor.
In the past, there have been numerous attempts to construct an accurate, small and safe intraocular pressure sensor. Among the devices proposed were direct cannulation of the anterior chamber of the eye coupled to an extraocular direct pressure monitor, and telemetric units using piezoresistive and acousto-optic elements. Such devices would be implanted in the anterior chamber either as free-standing units, or incorporated as parts of plastic intraocular lenses. The telemetric machines would transfer intraocular pressure readings to external monitoring devices non-invasively through the intact cornea. Although those previously proposed telemetric devices offer potential advantages over their invasive counterparts and the current indirect corneal devices, they still suffer many drawbacks including bulk, need for electrical power and unacceptable signal-to-noise ratios.
Recently, intracavity pressure sensors (e.g. brain and intravascular space) based upon the Fabry-Perot interferometer, in which two parallel, minimally separated, partially reflecting surfaces form an optical reflecting cavity, have been proposed. If one of the parallel surfaces is a pressure-sensitive diaphragm, changes in external pressure cause a change in the depth of the optical reflecting cavity, which in turn alters optical cavity reflectance spectra. Because brain and intravascular elements are optically opaque, current use requires a single wavelength light-emitting diode physically coupled to an input and read-out fiber optic. Alternatively, for the purposes of this current invention, we recognize that the anterior chamber and cornea are optically clear. Thus the input optical wavelengths and reflected output can be detected externally through intact and optically clear anterior chamber and cornea media after intraocular implantation of such a chip-based pressure sensor, either as an independent device or as part of an intraocular lens. In this case, because we are not restricted by the spectral bandpass of an optical fiber, almost any light source, including various LEDs, lasers or white light emitters (filtered and unfiltered) may be used. Moreover, currently available sensors should prove small enough for practical intraocular implantation. The advantages of direct intraocular pressure sensing, no need for electrical power, non-invasive external monitoring, compact chip-based device and optical sensing with high signal-to-noise ratio have been realized in this invention.
Measurement of Intraocular, Subconjunctival or Subdermal Analyte Concentrations
In clinical medicine, it is sometimes desirable to measure the concentration of glucose and/or other analytes within the eye or at other locations within the body, in order to diagnose and/or monitor various conditions including, but not limited to, metabolic or endocrine disorders such as such as diabetes mellitus. Various methods including direct analytical sampling and various forms of spectroscopy have been proposed in the past. Frequent direct invasive sampling, especially from the intraocular and intravascular spaces, has obvious problems. Non-invasive spectroscopic monitoring through skin and intravascular elements has sensitivity and specificity problems associated with both the optical opacity and turbidity of these media and the narrow (but often overlapping) spectroscopic chemical bands of each individual analyte.
Recently biomembranes permeable to specific analytes (e.g. glucose) have been developed. Sensors for these selected compounds usually incorporate direct spectroscopic detection or transduced increased pressure associated with increasing concentrations of the chemical. Such methods either involve invasive sampling of the sample chamber or electrical-powered piezoresistive signal transduction and read-out, all serious drawbacks of the proposed methods.
There remains a need in the art for the development of new devices and methods for measurement of intraocular pressure and/or measurement of intraocular, subconjunctival or subdermal analyte concentration.